Exchange 2010 Edge Transport Server STIG

U_Exchange_2010_Edge_V1R12_Manual-xccdf.xml

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V1R12

2017-01-03

Update existing CKLs to this version of the STIG

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The Microsoft Exchange Server 2010 STIGs cover four of the five roles available with Microsoft Exchange Server 2010. The Email Services Policy STIG must also be reviewed for each site hosting email services. Also, for the Client Access server, the IIS guidance must be reviewed prior to the OWA checks. Comments or proposed revisions to this document should be sent via e-mail to the following address: [email protected]

Vuln

Rule

Version

CCI

Severity

Title

Description

SV-43976r1_rule

Exch-2-334

MEDIUM

Sender Identification Framework must be enabled.

Email is only as secure as the recipient. When the recipient is an email server accepting inbound messages, authenticating the sender enables the receiver to better assess message quality and to validate the sending domain as authentic. One or more authentication techniques used in combination can be effective in reducing SPAM, PHISHING, and FORGERY attacks.
The Sender ID Framework (SIDF) receiver accesses specially formatted DNS records (SPF format) that contain the IP address of authorized sending servers for the sending domain that can be compared to data in the email message header. Receivers are able to validate the authenticity of the sending domain, helping to avoid receiving inbound messages from PHISHING or other SPAM domains.

SV-43977r1_rule

Exch-2-336

MEDIUM

SMTP Sender Filter must be enabled.

Email system availability depends in part on best practices strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts.
Filters that govern inbound Email evaluation can significantly reduce SPAM, PHISHING, and SPOOFED Emails. Messages from blank senders, known SPAMMERS, or 0-day attack modifications must be enabled to be effective.
Failure to enable the filter will result in no action taken. This setting should always be enabled.

SV-43978r1_rule

Exch-2-339

MEDIUM

SMTP IP Allow List Connection Filter must be enabled.

Email system availability depends in part on best practices strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts.
Filters that govern inbound Email evaluation can significantly reduce SPAM, PHISHING, and SPOOFED Emails. Messages from blank senders, known SPAMMERS, or 0-day attack modifications must be enabled to be effective.
Having items identified in the ‘allow’ list causes other SPAM evaluation steps to be bypassed, and therefore should be used only with an abundance of caution. If SPAMMERS were to learn of entries in the ‘allow list’ it could enable them to plan a denial of service attack (or other attack) by spoofing that source.

SV-43980r1_rule

Exch-2-342

MEDIUM

SMTP IP Allow List entries must be empty.

Email system availability depends in part on best practices strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts.
Filters that govern inbound Email evaluation can significantly reduce SPAM, PHISHING, and SPOOFED Emails. Messages from blank senders, known SPAMMERS, or 0-day attack modifications must be enabled to be effective.
Having items identified in the ‘allow’ list causes other SPAM evaluation steps to be bypassed, and therefore should be used only with an abundance of caution. If SPAMMERS were to learn of entries in the ‘allow list’ it could enable them to plan a denial of service attack (or other attack) by spoofing that source.

SV-43981r3_rule

Exch-2-705

LOW

Message size restrictions must be controlled on Receive connectors.

Email system availability depends in part on best practices strategies for setting tuning configurations. For message size restrictions, multiple places exist to set or override inbound or outbound message size. Failure to control the configuration strategy can result in loss of data or system availability.
This setting enables the administrator to control the maximum message size on receive connectors. Using connectors to control size limits may necessitate the need to apply message size limitations in multiple places, with the potential of introducing conflicts and impediments in the mail flow. Changing this setting at the connector overrides the global one. Therefore, if operational needs require it, the connector value may be set lower than that of the global value with the rationale and documented in the EDSP.

SV-43983r2_rule

Exch-2-708

LOW

Internet Receive Connector connections count must be set to default.

Email system availability depends in part on best practices strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous inbound connections allowed to the SMTP server.
By default, the number of simultaneous inbound connections is 5000. If a limit is set too low, the connections pool may get filled. If attackers perceive the limit is too low, they could deny service to the Simple Mail Transfer Protocol (SMTP) server by using a connection count that exceeds the limit set. By setting the default configuration to 5000, attackers would need many more connections to cause denial of service.

SV-43985r1_rule

Exch-2-710

LOW

Receive Connector timeout must be limited.

Email system availability depends in part on best practices strategies for setting tuning. This configuration controls the number of idle minutes before the connection is dropped. It works in conjunction with the Maximum Inbound Connections Count setting.
Connections, once established, may incur delays in message transfer. If the timeout period is too long, there is risk that connections may be maintained for unnecessarily long time periods, preventing new connections from being established.
ECSC-1

SV-43986r1_rule

Exch-2-715

MEDIUM

Internal Receive Connectors must not allow anonymous connections.

This control is used to limit the servers that may use this server as a relay. If a Simple Mail Transport Protocol (SMTP) sender does not have a direct connection to the Internet (for example, an application that produces reports to be emailed) then it will need to use an SMTP Receive Connector that does have a path to the Internet (for example, a local email server) as a relay.
SMTP relay functions must be protected so third parties are not able to hijack a relay service for their own purposes. Most commonly, hijacking of relays is done by SPAMMERS to disguise the source of their messages, and may also be used to cover the source of more destructive attacks.
Relays can be restricted in one of three ways; by blocking relays (restrict to a blank list of servers), by restricting use to lists of valid servers, or by restricting use to servers that can authenticate. Because authenticated connections are the most secure for SMTP Receive Connectors, it is recommended that relays allow only servers that can authenticate.ECSC-1

SV-43987r1_rule

Exch-2-718

MEDIUM

Internal Receive Connectors must require encryption.

The Simple Mail Transfer Protocol (SMTP) Receive Connector is used by Exchange to send and receive messages from server to server using SMTP protocol. This setting controls the encryption strength used for client connections to the SMTP Receive Connector. With this feature enabled, only clients capable of supporting secure communications will be able to send mail using this SMTP server. Where secure channels are required, encryption can also be selected.
The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from the client to the server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender.
Individually, channel security and encryption have been compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between the client and server.ECSC-1

SV-43988r1_rule

Exch-2-721

MEDIUM

External Receive Connectors must be Domain Secure Enabled.

The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. There are several controls that work together to provide security between internal servers. This setting controls the authentication method used for communications between servers. With this feature enabled, messages can be securely passed from a partner domain securely.
The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender.
Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.ECSC-1

Sending unencrypted email over the Internet increases the risk that messages can be intercepted or altered. Transport Layer Security (TLS) is designed to protect confidentiality and data integrity by encrypting email messages between servers and thereby reducing the risk of eavesdropping, interception, and alteration. This setting forces Exchange to offer TLS before using basic authentication.ECSC-1

SV-43992r2_rule

Exch-2-727

LOW

Receive Connectors must control the number of recipients per message.

Email system availability depends in part on best practices strategies for setting tuning configurations.
This configuration controls the maximum number of recipients who will receive a copy of a message at one time. This tunable value is related to throughput capacity and can enable the ability to optimize message delivery.
Note: There are two types of default Receive Connecters:
Client Servername: This Receive connector accepts SMTP connections from all non-MAPI clients, such as POP and IMAP. As POP and IMAP are not authorized for use in DoD, these should not be present. Their default value for MaxRecipientsPerMessage is 200.
Default Servername: This Receive connector accepts connections from other Hub Transport servers and any Edge Transport servers. Their default value for MaxRecipientsPerMessage is 5000.

SV-43994r1_rule

Exch-2-730

LOW

Receive Connectors must control the number of recipients chunked on a single message.

Email system availability depends in part on best practices strategies for setting tuning configurations. For message size restrictions, multiple places exist to set or override inbound or outbound message size. Failure to control the configuration strategy can result in loss of data or system availability.
This setting enables the administrator to enable ‘chunking’ on received messages as they arrive at the domain. This is done so that large message bodies can be relayed by the remote sender to the receive connector in multiple, smaller chunks.ECSC-1

SV-43995r1_rule

Exch-2-733

LOW

Receive Connectors must be clearly named.

For receive connectors, unclear naming as to direction and purpose increases risk that messages may not flow as intended, troubleshooting efforts may be impaired, or incorrect assumptions may be made about the completeness of the configuration.
Collectively, connectors should account for all connections required for the overall email topology design. Simple Mail Transfer Protocol (SMTP) connectors, when listed, must name purpose and direction clearly, and their counterparts on servers to which they connect should be recognizable as their partners.
ECSC-1

SV-43996r2_rule

Exch-2-736

MEDIUM

Auto-forwarding email to remote domains must be disabled or restricted.

Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Ensure Automatic Forwards to remote domains are disabled, except for enterprise mail that must be restricted to forward-only to .mil and .gov. domains.
Before enabling this setting first configure a remote domain.

SV-43998r1_rule

Exch-2-739

MEDIUM

Tarpitting interval must be set.

Tarpitting is the practice of artificially delaying server responses for specific SMTP communication patterns that indicate high volumes of SPAM or other unwelcome messages. The intent of tarpitting is to slow down the communication process for SPAM batches so that the cost effectiveness of sending SPAM is reduced and directory harvest attacks may be thwarted.
A directory harvest attack is an attempt to collect valid email addresses from a particular organization so that the email addresses can be added to a spam database. A program can be written to collect email addresses that return a 'Recipient OK' SMTP response and discards all email addresses that return a 'User unknown' SMTP response.
Tarpitting makes directory harvest attacks too costly to automate efficiently.ECSC-1

SV-43999r2_rule

Exch-2-741

LOW

Receive Connector Maximum Hop Count must be 60.

Email system availability depends in part on best practices strategies for setting tuning configurations. This setting controls the maximum number of hops (email servers traversed) a message may take as it travels to its destination. Part of the original Internet protocol implementation, the hop count limit prevents a message being passed in a routing loop indefinitely. Messages exceeding the maximum hop count are discarded undelivered.
Recent studies indicate that virtually all messages can be delivered in fewer than 60 hops. If the hop count is set too low, messages may expire before they reach their destinations. If set too high, an undeliverable message may cycle between servers, raising the risk of network congestion.

SV-44001r1_rule

Exch-2-743

MEDIUM

Recipient filter must be enabled.

Email system availability depends in part on best practices strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts.
Filters that govern inbound Email evaluation can significantly reduce SPAM, PHISHING, and SPOOFED Emails. Messages from blank senders, known SPAMMERS, or 0-day attack modifications must be enabled to be effective. ECSC-1

SV-44004r1_rule

Exch-2-751

LOW

Send Connectors must be clearly named.

For send connectors, unclear naming as to direction and purpose increases risk that messages may not flow as intended, troubleshooting efforts may be impaired, or incorrect assumptions may be made about the completeness of the configuration.
Collectively, connectors should account for all connections required for the overall email topology design. Simple Mail Transfer Protocol (SMTP) connectors, when listed, must name purpose and direction clearly, and their counterparts on servers to which they connect should be recognizable as their partners.ECSC-1

SV-44006r1_rule

Exch-2-754

LOW

Send Connectors delivery retries must be controlled.

This setting controls the rate at which delivery attempts from the home domain are retried, user notifications are issued, and notes the expiration time when the message will be discarded.
If delivery retry attempts are too frequent, servers will generate network congestion. If too far apart, then messages may remain queued longer than necessary, potentially raising disk resource requirements.
The default values of these fields should be adequate for most environments. Administrators may wish to modify the values as a result, but changes should be documented in the System Security Plan.
NOTE: Transport configuration settings apply to the organization/global level of the Exchange SMTP path. By checking and setting them at the Hub server the setting will apply to both Hub and Edge roles.ECSC-1

SV-44007r3_rule

Exch-2-757

LOW

Message size restrictions must be controlled on Send connectors.

Email system availability depends in part on best practices strategies for setting tuning configurations. For message size restrictions, multiple places exist to set or override inbound or outbound message size. Failure to control the configuration strategy can result in loss of data or system availability.
This setting enables the administrator to control the maximum message size on a send connector. Using connectors to control size limits may necessitate the need to apply message size limitations in multiple places, with the potential of introducing conflicts and impediments in the mail flow. Changing this setting at the connector overrides the global one. Therefore, if operational needs require it, the connector value may be set lower than that of the global value with the rationale and documented in the EDSP.

SV-44009r1_rule

Exch-2-760

LOW

Send Connector connections count must be limited.

This setting controls the maximum number of simultaneous outbound connections allowed for a given SMTP Connector, and can be used to throttle the SMTP service if resource constraints warrant it. If the limit is too low, connections may be dropped. If too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces risk of data delay or loss.
ECSC-1

The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. There are several controls that work together to provide security between internal servers. This setting controls the authentication method used for communications between servers. With this feature enabled, only servers capable of supporting domain authentication will be able to send and receive mail within the domain.
The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender.
Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.ECSC-1

SV-44012r3_rule

Exch-2-766

MEDIUM

Internal Send Connectors must require encryption.

The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. There are several controls that work together to provide security between internal servers. This setting controls the encryption method used for communications between servers. With this feature enabled, only servers capable of supporting Transport Layer Security (TLS) will be able to send and receive mail within the domain.
The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender.
Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.

SV-44014r2_rule

Exch-2-771

MEDIUM

Internet facing send Connectors must specify a Smart Host.

In the case of identifying a 'Smart Host' for the email environment, a logical send connector is the preferred method.
A 'Smart Host' acts as an Internet Facing Concentrator for other email servers. Appropriate hardening can be applied to the Smart Host rather than at multiple locations throughout the enterprise.
Failure to identify a 'Smart Host' could default to each email server performing its own lookups (potentially through protective firewalls). Exchange servers should not be Internet facing, and should therefore not perform any 'Smart Host' functions. When the Exchange servers are Internet facing they must, however, be configured to identify the Internet facing server that is performing the 'Smart Host' function.

SV-44016r1_rule

Exch-2-801

MEDIUM

Connectivity logging must be enabled.

A connectivity log is a record of the SMTP connection activity of the outbound message delivery queues to the destination Mailbox server, smart host, or domain. Connectivity logging is available on Hub Transport servers and Edge Transport servers. By default, connectivity logging is disabled. If events are not recorded it may be difficult or impossible to determine the root cause of system problems or the unauthorized activities of malicious users.
NOTE: Transport configuration settings apply to the organization/global level of the Exchange SMTP path. By checking and setting them at the Hub server the setting will apply to both Hub and Edge roles.ECSC-1

SV-44018r1_rule

Exch-2-805

MEDIUM

Exchange must not send delivery reports to remote domains.

Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Ensure that delivery reports to remote domains are disabled. Before enabling this setting first configure a remote domain using the EMC or the New-RemoteDomain cmdlet.ECSC-1

SV-44019r1_rule

Exch-2-808

MEDIUM

Exchange must not send non-delivery reports to remote domains.

Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Ensure that non-delivery reports to remote domains are disabled. Before enabling this setting first configure a remote domain using the EMC or the New-RemoteDomain cmdlet.ECSC-1

SV-44021r1_rule

Exch-2-811

MEDIUM

External/Internet bound automated response messages must be disabled.

SPAM originators, in an effort to refine mailing lists, sometimes use a technique where they monitor transmissions for automated bounce back messages, such as 'Out of Office' messages. Automated messages include such items as Out of Office responses, non-delivery messages, or automated message forwarding.
Automated bounce back messages can be used by a third party to determine if users exist on the server. This can result in the disclosure of active user accounts to third parties, paving the way for possible future attacks. ECSC-1

SV-44023r1_rule

Exch-2-814

MEDIUM

Exchange must not send auto replies to remote domains.

Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Remote users will not receive automated 'Out Of Office' delivery reports. This setting can be used to determine if all the servers in the Organization can send 'Out of Office' messages.ECSC-1

SV-44026r2_rule

Exch-2-817

MEDIUM

Email Diagnostic log level must be set to low or lowest level.

Log files help establish a history of activities, and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Diagnostic logging, however, characteristically produces large volumes of data and requires care in managing the logs to prevent risk of disk capacity denial of service conditions.
Exchange diagnostic logging is broken up into 29 main “services”, each of which has anywhere from 2 to 26 “categories” of events to be monitored. Moreover, each category may be set to one of four levels of logging: Lowest, Low, Medium, and High, depending on how much detail one desires. The higher the level of detail, the more disk space required to store the audit material.
Diagnostic logging is intended to help administrators debug problems with their systems, not as a general purpose auditing tool. Because the diagnostic logs collect a great deal of information, the log files may grow huge very quickly. Diagnostic log levels may be raised for limited periods of time when attempting to debug relevant pieces of Exchange functionality. Once debugging has finished, diagnostic log levels should be reduced again.

SV-44028r2_rule

Exch-2-820

MEDIUM

The Send Fatal Errors to Microsoft must be disabled.

Log files help establish a history of activities, and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. This setting enables an automated log entry to be sent to Microsoft giving general details about the nature and location of the error. Microsoft, in turn, uses this information to improve the robustness of their product.
While this type of debugging information would not ordinarily contain sensitive information, it may alert eavesdroppers to the existence of problems in your Exchange organization. At the very least, it could alert them to (possibly) advantageous timing to mount an attack. At worst, it may provide them with information as to which aspects of Exchange are causing problems and might be vulnerable (or at least sensitive) to attack.
All system errors in Exchange will result in outbound traffic that may be identified by an eavesdropper. For this reason, the 'Report Fatal Errors to Microsoft' feature must be disabled.

SV-44031r1_rule

Exch-2-826

MEDIUM

Audit data must be protected against unauthorized access.

Log files help establish a history of activities, and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive, and in need of protection. Audit data available for modification by a malicious user can be altered to conceal malicious activity. Audit data might also provide a means for the malicious user to plan unauthorized activities that exploit weaknesses.
The contents of audit logs are protected against unauthorized access, modification, or deletion. Only authorized auditors and the audit functions should be granted Read and Write access to audit log data.ECSC-1

SV-44033r1_rule

Exch-2-828

MEDIUM

Exchange application directory must be protected from unauthorized access.

Default product installations may provide more generous access permissions than are necessary to run the application. By examining and tailoring access permissions to more closely provide the least amount of privilege possible, attack vectors that align with user permissions are less likely to access more highly secured areas.ECSC-1

SV-44036r1_rule

Exch-2-831

MEDIUM

Exchange must not send Customer Experience reports to Microsoft.

Log files help establish a history of activities, and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. This setting enables an automated entry to be sent to Microsoft giving general details about how the product is used. Microsoft, in turn, uses this information to improve the robustness of their product.
While this type of information does not ordinarily contain sensitive information, it may alert eavesdroppers to the existence of the environment and its configurations. It could alert them to (possibly) advantageous timing or weaknesses toward which to mount an attack.
ECSC-1

SV-44038r1_rule

Exch-2-839

MEDIUM

Audit data must be on separate partitions.

Log files help establish a history of activities, and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive, and in need of protection.
Successful exploit of an application server vulnerability may well be logged by monitoring or audit processes when it occurs. By writing log and audit data to a separate partition where separate security contexts protect them, it may offer the ability to protect this information from being modified or removed by the exploit mechanism.ECSC-1

SV-44039r3_rule

Exch-2-842

MEDIUM

Queue monitoring must be configured with threshold and action.

Monitors are automated “process watchers” that respond to performance changes, and can be useful in detecting outages and alerting administrators where attention is needed. Exchange has built-in monitors that enable the administrator to generate alerts if thresholds are reached, better enabling them to react in a timely fashion.
The intent of this check is for system administrators to have awareness of performance changes on their network.
Notification choices include email an alert to an email-enabled account, for example, an email Administrator, or invoke a script to take other action, for example, to add an Event to the Microsoft Application Event Log, where external monitors might detect it.
Data elements configured to be monitored should be specific to the organization’s network.
.

SV-44040r1_rule

Exch-3-003

MEDIUM

Email software must be monitored for change on INFOCON frequency schedule.

The INFOCON system provides a framework within which the Commander USSTRATCOM regional commanders, service chiefs, base/post/camp/station/vessel commanders, or agency directors can increase the measurable readiness of their networks to match operational priorities. The readiness strategy provides the ability to continuously maintain and sustain one’s own information systems and networks throughout their schedule of deployments, exercises, and operational readiness life cycle independent of network attacks or threats. The system provides a framework of prescribed actions and cycles necessary for reestablishing the confidence level and security of information systems for the commander and thereby supporting the entire Global Information Grid (GIG) (SD 527-1 Purpose).
The Exchange software files and directories are vulnerable to unauthorized changes if not adequately protected. An unauthorized change could affect the integrity or availability of email services overall. For this reason, all application software installations must monitor for change against a software baseline that is preserved when installed, and updated periodically as patches or upgrades are installed. Automated and manual schedules for software change monitoring must be compliant with SD527-1 frequencies.
Note: Policy Auditor 5.2 or later, File Integrity Monitor (FIM) module will meet the requirement for file integrity checking. The Asset module within HBSS does not meet this requirement.
ECSC-1

SV-44041r1_rule

Exch-3-006

MEDIUM

Exchange software baseline copy must exist.

Exchange software, as with other application software installed on a host system, must be included in a system baseline record and periodically reviewed; otherwise unauthorized changes to the software may not be discovered. This effort is a vital step to securing the host and the applications, as it is the only method that may provide the ability to detect and recover from otherwise undetected changes, such as those that result from worm or bot intrusions.
The Exchange software and configuration baseline is created and maintained for comparison during scanning efforts. Operational procedures must include baseline updates as part of configuration management tasks that change the software and configuration.
ECSC-1

SV-44042r1_rule

Exch-2-005

MEDIUM

Accepted domains must be configured.

Exchange may be configured to accept email for multiple domain names. This setting identifies the domains for which the server will accept mail. This check verifies the email server is not accepting email for unauthorized domains.ECSC-1

SV-44043r2_rule

Exch-3-804

MEDIUM

Services must be documented and unnecessary services must be removed or disabled.

Unneeded, but running, services offer attackers an enhanced attack profile, and attackers are constantly watching to discover open ports with running services. By analyzing and disabling unneeded services, the associated open ports become unresponsive to outside queries, and servers become more secure as a result.
Exchange Server has role-based server deployment to enable protocol path control and logical separation of network traffic types.
For example, a server implemented in the Client Access role (i.e., Outlook Web App [OWA]) is configured and tuned as a web server using web protocols. A client access server exposes only web protocols (HTTP/HTTPS) enabling system administrators to optimize the protocol path and disable all services unnecessary for Exchange web services. Similarly, servers created to host mailboxes are dedicated to that task, and must operate only the services needed for mailbox hosting. (Exchange servers must also operate some Web services, but only to the degree that Exchange requires the IIS engine in order to function).
Because POP3, and IMAP4 clients are not included in the standard desktop offering, they must be disabled.

SV-44044r1_rule

Exch-2-011

LOW

Global inbound message size must be controlled.

Email system availability depends in part on best practices strategies for setting tuning configurations. Message size limits should be set to 10 megabytes at most, but often are smaller, depending on the organization. The key point in message size is that it should be set globally, and it should not be set to ‘unlimited’. Selecting ‘unlimited’ on either field is likely to result in abuse and can contribute to excessive server disk space consumption.
Message size limits may also be applied on SMTP connectors, Public Folders, and on the user account under AD. Changes at these lower levels are discouraged, as the single global setting is usually sufficient. This practice prevents conflicts that could impact availability and it simplifies server administration.
ECSC-1

SV-44045r2_rule

Exch-3-807

MEDIUM

Email application must not share a partition with another application.

In the same way that added security layers can provide a cumulative positive effect on security posture, multiple applications can provide a cumulative negative effect. A vulnerability and subsequent exploit to one application can lead to an exploit of other applications sharing the same security context. For example, an exploit to a web server process that leads to unauthorized administrative access to the host system can most likely lead to a compromise of all applications hosted by the same system.
Email services should be installed on a partition that does not host other applications. Email services should never be installed on a Domain Controller / Directory Services server.

SV-44046r2_rule

Exch-2-014

MEDIUM

Servers must use approved DoD certificates.

Server certificates are required for many security features in Exchange; without them the server cannot engage in many forms of secure communication.
Failure to implement valid certificates makes it virtually impossible to secure Exchange's communications.

SV-44047r2_rule

Exch-2-015

LOW

Global outbound message size must be controlled.

Email system availability depends in part on best practices strategies for setting tuning configurations. Message size limits should be set to 10 megabytes at most, but often are smaller, depending on the organization. The key point in message size is that it should be set globally, and it should not be set to ‘unlimited’. Selecting ‘unlimited’ on either field is likely to result in abuse and can contribute to excessive server disk space consumption.
Message size limits may also be applied on send and receive connectors, Public Folders, and on the user account under AD. Changes at these lower levels are discouraged, as the single global setting is usually sufficient. This practice prevents conflicts that could impact availability and it simplifies server administration.

SV-44049r3_rule

Exch-3-814

MEDIUM

The current, approved service pack must be installed.

Failure to install the most current Exchange service pack leaves a system vulnerable to exploitation. Current service packs correct known security and system vulnerabilities.

SV-44050r1_rule

Exch-2-017

LOW

Global recipient count limit must be set.

Email system availability depends in part on best practices strategies for setting tuning configurations. The Global Recipient Count limit field is used to control the maximum number of recipients that can be specified in a single message sent from this server. Its primary purpose is to minimize the chance of an internal sender spamming other recipients, since SPAM messages often have a large number of recipients. SPAM prevention can originate from both outside and inside organizations. While inbound SPAM is evaluated as it arrives, controls such as this one help prevent SPAM that might originate inside the organization.
The Recipient Count Limit is global to the Exchange implementation. Lower-level refinements are possible; however, in this configuration strategy, setting the value once at the global level ensures a more available system by eliminating potential conflicts among multiple settings. A value of less than or equal to 5000 is probably larger than is needed for most organizations, but is small enough to minimize usefulness to spammers, and is easily handled by Exchange. An unexpanded distribution is handled as one recipient. Specifying “unlimited” may result in abuse.
ECSC-1

SV-44051r1_rule

Exch-2-333

MEDIUM

Messages with malformed from address must be rejected.

Sender Identification (SID) is an email anti-spam sanitization process. Sender ID uses DNS MX record lookups to verify the SMTP sending server is authorized to send email for the originating domain.
Failure to implement Sender ID risks that SPAM could be admitted into the email domain that originates from rogue servers. Most SPAM content originates from domains where the IP address has been spoofed prior to sending, thereby avoiding detection. For example, messages with malformed or incorrect 'purported responsible sender' data in the message header could be (best case) created by using RFI non-compliant software, but is more likely to be SPAM.
ECSC-1

SV-44052r1_rule

Exch-2-019

MEDIUM

Local machine policy must require signed scripts.

Scripts often provide a way for attackers to infiltrate a system, especially those downloaded from untrusted locations. By setting machine policy to prevent unauthorized script executions, unanticipated system impacts can be avoided. Failure to allow only signed remote scripts reduces the attack vector vulnerabilities from unsigned remote scripts. ECSC-1

SV-44053r2_rule

Exch-2-330

MEDIUM

Block list service provider must be identified.

Block List filtering is a sanitization process performed on email messages prior to their arrival at the destination mailbox. By performing this process at the email perimeter, threats can be eliminated outside the enclave, where there is less risk they can do harm.
Block List Services (sometimes called Reputation Data Services) are fee based data providers that collect the IP addresses of known SPAMmers and other malware purveyors. Block List Service Subscribers benefit from more effective SPAM elimination, which has been estimated as comprising up to 90% of inbound mail volume. Failure to specify a Block List provider risks that manual email administration effort would be needed to maintain and update larger block lists than a single email site administrator could conveniently or accurately maintain.
The 'Block List' Services vendor provides a value for this field, usually the DNS suffix for their domain.

SV-44054r1_rule

Exch-2-200

MEDIUM

SMTP automated banner response must not reveal server details.

Automated connection responses occur as a result of FTP or Telnet connections, when connecting to those services. They report a successful connection by greeting the connecting client, stating the name, release level, and (often) additional information regarding the responding product. While useful to the connecting client, connection responses can also be used by a third party to determine operating system (OS) or product release levels on the target server. The result can include disclosure of configuration information to third parties, paving the way for possible future attacks. For example, when querying the SMTP service on port 25, the default response looks similar to this one:
220 exchange.mydomain.org Microsoft ESMTP MAIL Service, Version: 6.0.3790.211 ready at Wed, 2 Feb 2005 23:40:00 -0500
Changing the response to hide local configuration details reduces the attack profile of the target.
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SV-44055r2_rule

Exch-2-201

LOW

Outbound Connection Limit per Domain Count must be controlled.

Email system availability depends in part on best practices strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous outbound connections from a domain, and works in conjunction with the Maximum Outbound Connections Count setting as a delivery tuning mechanism. If the limit is too low, connections may be dropped. If too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces risk of data delay or loss.
By default, a limit of 20 simultaneous outbound connections from a domain should be sufficient. The value may be adjusted if justified by local site conditions.

SV-44056r1_rule

Exch-2-327

MEDIUM

SPAM evaluation filter must be enabled.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages may be eliminated from the transport message stream, preventing their entry into the Exchange environment. This significantly reduces the attack vector for inbound email-borne SPAM and malware.
SPAM evaluation (heuristic) filters scan inbound email messages for evidence of SPAM and other attacks that primarily use 'Social Engineering' techniques. Upon evaluation completion, a rating is assigned to each message estimating the likelihood of its being SPAM. Upon arrival at the destination mailbox, the junk mail filter threshold (also configurable) determines whether the message will be withheld from delivery, delivered to the junk mail folder, or delivered to the user’s inbox.
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By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Attachments are being used more frequently for different forms of attacks. By filtering undesirable attachments a large percent of malicious code can be prevented from entering the system. Attachments must be controlled at the entry point into the email environment to prevent successful attachment-based attacks. The following is a basic list of known attachments that should be filtered from Internet mail attachments.
*.ade *.crt *.jse *.msi *.scr *.wsh *.dir *.adp *.csh *.ksh *.msp *.sct *.htm *.dcr *.app *.exe *.lnk *.mst *.shb *.html *.plg *.asx *.fxp *.mda *.ops *.shs *.htc *.spl *.bas *.hlp *.mdb *.pcd *.url *.mht *.swf
*.bat *.hta *.mde *.pif *.vb *.mhtml *.chm *.inf *.mdt *.prf *.vbe *.shtm *.cmd *.ins *.mdw *.prg *.vbs *.shtml
*.com *.isp *.mdz *.reg *.wsc *.stm
*.cpl *.js *.msc *.scf *.wsf

SV-44058r1_rule

Exch-2-324

MEDIUM

Sender reputation filter must identify SPAM block level.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Sender reputation is anti-SPAM functionality that blocks messages according to many characteristics of the sender. Sender reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message. This setting enables the threshold at which an email will be considered SPAM.ECSC-1

SV-44059r1_rule

Exch-2-321

MEDIUM

Sender reputation filter must be enabled.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the Mail server environment. Sender reputation is anti-SPAM functionality that blocks messages according to many characteristics of the sender. Sender reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message. This setting enables the sender reputation function.ECSC-1

SV-44060r1_rule

Exch-2-305

MEDIUM

Non-existent recipients must not be blocked.

SPAM originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names, and then monitor rejected emails for non-existent recipients.
Those not rejected, of course, are deemed to exist, and are therefore used in future SPAM mailings.
To prevent this disclosure of existing email accounts to Spammers, this feature should not be employed. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine recipients that are existing vs. non-existing.
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SV-44061r1_rule

Exch-2-317

MEDIUM

Sender Filter must block accepted domains at the edge.

SPAM origination sites and other sources of suspected Email-borne malware have the ability to corrupt, compromise, or otherwise limit availability of Email servers. Limiting exposure to unfiltered inbound messages can reduce the risk of SPAM and malware impacts.
The Global Deny list block messages originating from specific sources. Most Black List filtering is done using a commercial 'Block List' service, because eliminating threats from known SPAMMERS prevents the messages being evaluated inside the enclave where there is more risk they can do harm.
Additional sources should also be blocked to supplement the contents of the commercial 'Block List Service'. For example, during a 0-Day threat action, entries can be added, and then removed when the threat is mitigated. An additional best practice is to enter the enterprise’s home domains in the Deny List, because inbound Email with a ‘from’ address of the home domain is very likely to be SPOOFED SPAM. ECSC-1

SV-44062r1_rule

Exch-2-308

MEDIUM

Filtered messages must be archived.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. This significantly reduces the attack vector for inbound Email-borne SPAM and malware.
As messages are filtered, it is prudent to temporarily host them in an archive for evaluation by administrators or users. The archive can be used to recover messages that might have been inappropriately filtered, preventing data loss, and to provide a base of analysis that can provide future filter refinements.
ECSC-1

SV-44063r1_rule

Exch-2-314

MEDIUM

Messages with blank sender field must be filtered.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Anonymous email (messages with blank sender fields) cannot be replied to. Messages formatted in this way may be attempting to hide their true origin to avoid responses, or to SPAM any receiver with impunity while hiding their source of origination.
Rather than spend resource and risk infection while evaluating them, it is recommended that these messages be filtered immediately upon receipt and not forwarded to end users.ECSC-1

SV-44064r1_rule

Exch-2-311

MEDIUM

Messages with blank senders must be rejected.

By performing filtering at the perimeter, up to 90% of SPAM, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Anonymous Email (messages with blank sender fields) cannot be replied to. Messages formatted in this way may be attempting to hide their true origin to avoid responses, or to SPAM any receiver with impunity while hiding their source of origination.
Rather than spend resource and risk infection while evaluating them, it is recommended that these messages be filtered immediately upon receipt and not forwarded to end users.
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SV-44066r1_rule

Exch-2-769

LOW

Outbound Connection Timeout must be 10 or less.

Email system availability depends in part on best practices strategies for setting tuning configurations. This configuration controls the number of idle minutes before the connection is dropped. It works in conjunction with the Maximum Outbound Connections Count setting.
Connections, once established, may incur delays in message transfer. The default of 10 minutes is a reasonable window in which to resume activities without maintaining idle connections for excessive intervals. If the timeout period is too long, idle connections may be maintained for unnecessarily long time periods, preventing new connections from being established. Sluggish connectivity increases the risk of lost data. A value of 10 or less is optimal.
ECSC-1

SV-75445r1_rule

Exch-2-768

MEDIUM

Internal Send Connectors must use an authentication level

The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. There are several controls that work together to provide security between internal servers. This setting controls the encryption method used for communications between servers. With this feature enabled, only servers capable of supporting Transport Layer Security (TLS) will be able to send and receive mail within the domain.
The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender.
Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.